The present invention relates to a measurement circuit for an electric measurement transmitter and an indicating instrument which has a display which is dependent on a transmitter voltage of the measurement transmitter, having means for suppressing variations of the display in a middle measurement range of the indicating instrument, particularly for monitoring the temperature of the cooling water of automobile internal combustion engines with a bimetallic indicating instrument and an NTC-resistor as measurement transmitter.
In known measurement circuits, a measurement transmitter, particularly an NTC-resistor (negative temperature coefficient resistor) arranged in series with an indicating instrument, is connected to a source of operating voltage. The NTC-resistor causes a current which varies in accordance with the ambient temperature of the NTC-resistor to pass through the indicating instrument, the current being relatively high when the temperature is high. By means of the indicating instrument a reading which is a function of the current or of the transmitter voltage on the NTC-resistor is obtained in a form of the deflection of a pointer.
If the cooling water temperature of an automotive internal combustion engine is to be monitored by the use of this measurement transmitter and the indicating instrument, it may be desirable to avoid indication of brief changes in temperature of the cooling water in operation since they occur even upon trouble-free operation of the automotive internal combustion engine, and might needlessly annoy the driver. On the other hand, the driver should be warned as soon as possible by a large deflection of the pointer should the temperature of the cooling water assume values which give reason to fear damage to the automotive internal combustion engine.
In order to attenuate undesired variations of the indication of the indicating instrument one could contemplate providing a relatively large damping within the indicating instrument itself or in the measurement circuit within which the indicating instrument is located. For example, the bimetallic indicating instruments generally used for the monitoring of the temperature of the cooling water already have substantial damping, which, however, is still not sufficient to adequately suppress the said large brief variations within a normal temperature range. Increasing the damping may not only result in increased expense for the storage of energy in the measurement circuit but, in particular, also result in a greatly delayed deflection of the pointer. It is important rapidly and clearly to indicate impermissibly high temperatures of the cooling water.